The present invention relates generally to heat pipes, and more specifically to a novel predominantly unidirectional wick that is particularly suited for use in heat pipes.
Heat pipes use successive evaporation and condensation of a working fluid to transport thermal energy, or heat, from a heat source to a heat sink. Because most fluids have a high heat of vaporization, heat pipes can transport in a vaporized working fluid very large amounts of heat. Advantageously, the heat can be transported over very small temperature differences between the heat source and heat sink. Heat pipes generally use capillary forces through a porous wick to return condensed working fluid, or condensate, from a heat pipe condenser section (where transported thermal energy is given up to the heat sink) to an evaporator section (where the thermal energy to be transported is absorbed from the heat source).
Heat pipes generally transfer heat equally well in either direction. It is desired in many applications, however, that the flow of heat be restricted in one direction so that, for example, heat may be stored and not lost if the temperature of the intended heat source drops below the temperature of the intended heat sink. Alternatively, in many applications it is desired to remove harmful heat from equipment and to protect the equipment from absorbing heat if the temperature of the intended heat sink rises above the temperature of the equipment. These requirements are met by the use of unidirectional heat pipes which transfer heat preferentially in one direction.
Unidirectional heat pipes transfer heat preferentially in one direction by several methods. In some cases, they simply transfer heat at a higher flow rate in one direction than in the other. In other cases, they will transfer only a limited amount of heat in one direction before preventing further heat flow in that direction.
A common method, or approach, for making unidirectional heat pipes, sometimes called thermal diodes, is by so-called liquid flow control techniques using liquid traps and liquid blockages. When operated in reverse from its intended or normal direction, a liquid trap thermal diode "traps" the liquid working fluid in a compartment adjacent to the normally evaporator, now condenser, section so that the wick dries out and heat pipe operation ceases. A liquid blockage thermal diode stores excess liquid working fluid next to the normal condenser section. When operated in reverse from its normal direction, the excess liquid working fluid collects in the normally evaporator, now condenser, section, so that the excess liquid "blocks" normal operation of the heat pipe.
Another common approach for making unidirectional heat pipes, instead of controlling generally the liquid flow, controls by various methods the vapor flow.
A third approach for making unidirectional heat pipes uses the wick to control the movement of liquid working fluid. One example of this approach teaches using a dual section wick having a thicker wick with smaller pores in the evaporator section, and a thinner wick with larger pores in the condenser section. When operated in reverse, the thin wick-large pore, now evaporator, wick section quickly dries out and prevents further heat flow.
While using the wick structure to make heat pipes unidirectional provides advantages of simplicity and easy retrofit to high performance prior art heat pipe designs, prior art attempts to provide such wicks have largely produced wicks of complex structure, requiring complex fabrication techniques, that are generally as difficult to incorporate in a heat pipe application as more elaborate liquid and vapor flow methods.
Thus it is seen that there is a need for a method for making unidirectional wicks for heat pipes that have a simple structure and are straightforward to make.
It is, therefore, a principal object of the present invention to provide a method for making a predominantly unidirectional wick for heat pipes that has a simple structure and is straightforward to make.
It is another object of the invention to provide a structure and method for making an improved sintered metal wick structure of arbitrary shape that wicks fluid preferentially in one direction.
A feature of the present invention is that it is easy to fabricate within prior art heat pipes, particularly by using the spinning pipe method for making improved sintered metal heat pipe wicks taught in companion applications Ser. No. 07/261,809. A Method of Manufacturing Heat pipe Wicks, and Ser. No. 07/261,807. A Method of Manufacturing Heat pipe Wicks and Arteries.